In harness racing, and other uses of draft animals, it is common practice to use a bridle having a bit for location in the mouth of the horse. Control of the head of the horse is achieved by pulling reins attached to the bridle, which pull the bit further into the mouth and also pulls the head of the horse downwards.
In situations where a horse is difficult to control the rider may pull on the reins with such force that the bit may damage the mouth and teeth. Jerking and bad driving may have a similar effect. The presence of the bit in the mouth may also result in breathing problems, choking, and even swallowing of the tongue. In addition, as mentioned above, pulling on the reins tends to pull the head downwards. Occasions have arisen where the heads of horses involved in harness races have been pulled downwards to such an extent that the horses have tripped and fallen, resulting in severe, and occasionally fatal, injury to the horses and riders.
Checks have therefore been used to help prevent the head of a difficult to control horse from being pulled downwards, by the driver pulling on the reins or by the horse itself. Such checks are commonly in the form of a leather loop which is passed over the forward end of the head and attached to a fixed point on a harness by an adjustable strap which passes over the head and between the ears. However, as a horse pulls downwardly against such a check, the loop tends to deform to constrict around the mouth and may interfere with the bit of the bridle. Also, the lips and skin of the horse tend to become caught between the loop and the bit and may be damaged.